The overall goal of this project is to explore the role of protein carboxyl methylation in regulation of synaptic and neuronal function. Carboxyl methylation is believed to play an important role in modulation of protein function, particularly in neural and endocrine tissues. A widely accepted model views carboxyl methylation as a reversible modification of protein function analogous to phosphorylation-dephosphorylation, yet there are several inconsistencies with this simple model. It has recently been found that carboxyl methylation is not a reversible reaction, but rather that it is the first step in a complex reaction which releases an unmethylated peptide that is different from the original substrate and cannot be remethylated. This finding has important implications for the in vivo function of carboxyl methylation and suggests that a new approach to assaying this enzyme is needed to properly evaluate its role in regulatory events. One immediate aim of the current proposal is to determine the nature of the unusual peptide modification catalyzed by protein carboxyl methyltransferase. Another aim is to use a methanol formation assay to search for endogenous substrates for this enzyme in subfractions of brain tissue and to evaluate the possibility that this reaction is modulated by transmembrane signals such as depolarization and neurotransmitter receptor activation, or by intracellular signals such as cAMP, cGMP and Ca++. This research sould substantially improve our understanding of the biochemical mechanisms which underly regulation of neuronal and synaptic activity in the mammalian brain.